Unit7 Shielded Gas Arc Welding

J3103/7/1 SHIELDED GAS ARC WELDING

General Objective: To understand the principles of shielded gas arcwelding i.e. TIG and MIG welding.

Specific Objectives : At the end of the unit you will be able to :

Ø Identify the principles of shielded gas arcwelding i.e. TIG and MIG welding.

Ø Elaborate on the TIG and MIG weldingprinciples, welding procedures, weldingmachines, gas, etc.

Ø State the advantages and disadvantages of TIGand MIG compared to manual arc welding.

Ø State the weaknesses of TIG and MIG weldingand how to prevent them.

.

UNIT 7

OBJECTIVES

SHIELDED GAS ARC WELDING

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7.0. INTRODUCTION

The objective of welding is to produce a welding joint that contains thesame mechanical properties as the base metal. The objective can be achieved

if the molten metal is free from atmospheric air. If not, nitrogen and oxygengases in the atmosphere will be absorbed by the melting pool. The weldingproduced will have small pore that will weaken the weld.

To prevent the welding, molten metal and the end of the filler rode andelectrodes from atmospheric air pollution before the molten metal becomesolid inert gas is blown out from the welding point. These gases will coverthe welding pools, the filler rod points and electrode tips to avoid oxidation.

7.1. TUNGSTEN INERT GAS (TIG)

The welding of aluminium and magnesium alloys by the oxy-acetyleneand manual metal arc processes is limited by the necessity to use a corrosiveflux. The gas shielded, tungsten arc process enables these metals and a wide

range of ferrous alloys to be welded without the use of a flux. The choice ofthe either a.c. or d.c. depends upon the metal to be welded. For metalshaving refractory surface oxides such as aluminium and its alloys,magnesium alloys and aluminium bronze, a.c. is used whilst d.c. is used for

INPUT

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carbon and alloy steels, heat-resistant and stainless steels, cooper and itsalloys, nickel and its alloys, titanium, zirconium and silver.

The arc burns between a tungsten electrode and the work piece within

a shield of the inert gas argon, which excludes the atmosphere and preventscontamination of electrode and molten metal. The hot tungsten arc ionizesargon atoms within the shield to form a gas plasma consisting of almostequal numbers of free electrons and positive ions. Unlike the electrode in themanual metal arc process, the tungsten is not transferred to the work andevaporates very slowly, being classed as ‘non-consumable’. Small amount of

other elements are added to the tungsten to improve electron emission.

Figure 7.1. TIG welding equipment

Gas flow

Water inlet

Water outlet Weldingmachine

Torch

Work piece

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7.1.1. Preparation of Metal.

Gas tungsten-arc processes must start with clean metal which

has the proper joint design i.e., V, U, or J. Mechanical and chemicalcleaning are often necessary to prepare the base metal. The edges ofthe joint should be shaped to permit adequate fusion and penetration.It is common practice to reduce or bevel the adjoining edges to 1.6 mmthickness.

A strip (backup bar) to support the back side of the base metal

should be used when needed. This is especially helpful on aluminiumsince it aids in shielding. The backup bar may be removed afterwelding.

Figure 7.2. TIG in progress. The tungsten does not melt into thepuddle for filler. This is a nonconsumable electrode.

Shielded gas

Electrode(tungsten)

Filler rode

arc

Melting pool

Inert/noblegas

Work piece

20 – 30o 80 – 90oDirection of travel

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7.1.2. Joint Fit.

Good joints make it easier to obtain a good weld. In production

work, carefully fitted joints can help save money and can help thewelding operator develop standardized welding techniques. Rootopening (distance apart) and angle of bevel are two major factorsrequiring close tolerance when fitting joints.

7.1.3. Welding Machine.

Gas tungsten-arc welding requires a conventional weldingmachine, with the following accessories:

1. Torch, lead cable, and hoses.2. Inert gas supply and flow meter for measuring amount

of shielding gas.

3. Water cooling system for water-cooled torches. Air-cooled torches are limited to 150 ampere capacity.

4. High-frequency spark unit attached to the output leadsof the power supply (to start and stabilize arc).

The finished weld will be greatly affected by type of current and

polarity. For example, aluminium is welded with alternating currentplus superimposed high-frequency current (ACHF). Stainless steel iswelded with direct current straight polarity (DCSP). Improperelectrical connections will cause (a) the electrode to overheat, (b) poorpenetration, or (c) insufficient cleaning effect upon the base metal.

Current selection must be made with care. When an electrode is

connected to the negative terminal (DCSP), electrons pass through thearc to bombard the base plate (Fig. 7.3).

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This causes nearly 70% of the arc heat to accumulate in thebase metal to assist fusion and penetration. When the electrode ismade positive (DCRP), a cleaning effect is created on the surface of thebase plate (Fig. 7.4).

In welding aluminium this method is used to remove surface

oxidation. While an electrode positive connection furnishes a cleaningeffect, it also heats the tungsten electrode. The electrode may get hot

Deep penetration

Work piece

Figure 7.3 Power supply with direct current straight polarity

Direction of electrontravel

Weldingmachine

Positive surfaceparticles travel

Electrode

Positive surfaceparticles travel

Direction of electrontravel

ElectrodeWeldingmachine

Work piece

Shallow penetration

Figure 7.4 Power supply with direct current reverse polarity

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enough to melt, transfer to the weld pool, and contaminate the basemetal. When this happens, the electrode must be removed, its endbroken off, and it must be ground to shape.

Alternating current offers the advantages of both direct currentstraight polarity (DCSP) and direct current reverse polarity (DCRP).Gas tungsten-arc welding of aluminium and magnesium requires anAC power supply (Fig. 7.5).

Gas tungsten-arc welding is not recommended for metal morethan 20 mm thick. Welds have been completed on 25 mm thick plate

but require a great deal of time and, consequently, are expensive.Most applications are less than 12 mm thick, and require less than 500amperes of current.

Electrode

Surfaceparticles lifted Electron flow

Weldingmachine

Work piece

Medium penetration

Figure 7.5 Alternating current power supply

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7.1.4. Welding Torch.

The welding torch has a round collet which compresses to hold

the electrode and a nozzle to control the gas (Fig. 7.2). Water-cooledtorches are used when current values exceed 150 amperes.Maintenance of either torch is more time consuming than with themetal-arc process. Careful selection of nozzle size, proper shaping ofthe working end of the electrode and correct extension of electrodebeyond nozzle are important. Nozzle size influences the flow of gas.

End shape of electrode and extension of electrode beyond nozzle controlthe stability of the arc. Further, it is important that electrode diametermatch current value (Table 7.1). If the current is too high for thediameter of an electrode, the life of the electrode will be reduced. Whenthe current is too low for a given electrode diameter, the arc will not bestable.

WELDING CURRENT IN AMPERES

ACHF DCSP DCRP

ElectrodeSize

(Diameter,Inches)

Nozzle orCup Sizes

Pure

Tungsten

Thoriated

Tungsten

Pure or

Thoriated

Pure or

Thoriated

0.020 4,5 5-15 5-20 5-20 *

0.040 4,5 10-60 15-80 15-80 *

1/16 4-6 50-100 70-150 70-150 10-20

3/32 5-7 100-160 140-235 150-250 15-30

1/8 6-8 150-210 225-325 250-400 25-40

*Not applicable.

Table 7.1. Selection of nozzle size and electrode size for gas tungsten-arcwelding

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The end of the electrode should remain bright, as if it waspolished. On some metals, such as aluminium and magnesium, the endis contaminated when starting or by touching the base plate.

Contamination can be burned off by welding on a scrap plate of metal,or it can be removed by grinding (Fig. 7.6). The electrode should beadjusted to extend beyond the nozzle a distance equal to the electrodediameter (Fig. 7.7)

Figure 7.6 Electrode shapes for gas shielded tungsten-arc welding

3/8” max

Electrode diameter

Figure 7.7. Adjustment of electrode from nozzle

Grind here

AC

30o

45o

15o

DCSP DCRP

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7.1.5. Shielding Gas.

Gas used with this process produces an atmosphere free from

contamination and also provides a path for arc transfer. The pathcreates an environment that helps stabilize the arc. The gas and arcactivity also perform a cleansing action on the base metal. Both argonand helium are generally used for this process but argon is preferredbecause it is cheaper and provides a smoother arc. Helium, however,helps produce deeper penetration (Table 7-2).

7.1.6. Filler Metal.

Filler metals are selected to meet or exceed the tensile strength,ductility, and corrosion resistance of the base metal. The usual practiceis to select a filler metal having a composition similar to that of the

base metal. For most efficient application, select clean filler metals ofproper diameter; the larger the diameter of the filler metal, the moreheat is lost from the weld pool.

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Metal Shielding Gas Remarks

Aluminium Argon Easy startingGood cleaning action.

Helium Faster and more penetration.

Argon-10% helium Increase in penetration over pure argon.

Stainless steel Argon Better control of penetration (16 gaugeand thinner).

Argon-heliummixtures

Higher welding speeds.

Copper andnickel

Argon Easy to control penetration and weldcontour on sheet metal.

Argon-helium Increases heat into base metal.

Helium Highest welding speed.

7.2. TIG WELDING TECHNIQUES

After the base metal has been properly cleaned and clamped or tacked

together, welding can be started. On aluminium, the arc is usually started bybringing the electrode near the base metal at a distance of about oneelectrode diameter so that a high-frequency spark jumps across the gap andstarts the flow of welding current. Steel, copper alloys, nickel alloys, andstainless steel may be touched with the electrode without contamination tostart the arc. Once started, the arc is held stationary until a liquid pool

appears. Filler rod can be added to the weld pool as required (Fig. 7.8).Highest current values and minimum gas flow should be used to produceclean, sound welds of desired penetration (Table 7-3).

Table 7.2 Selection of gases for manual application of tungsten-arc welding.

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Material Aluminium Stainless Steel Magnesium DeoxidizedCopper

Type of Current ACHF DCSP ACHF DCSP

1.6mm electrodeCurrent:Argon:Passes:

60-8015 cfh

1

80-10011 cfh

1

6013 cfh

1

110-14015 cfh

1


125-14517 cfh

1

120-14011 cfh

1

11519 cfh

1

175-22515 cfh

1


190-22021 cfh

1

200-25013 cfh

1

120-17519 cfh

1,2

250-30015 cfh

1 at 257.4*

*Preheat to temperature indicated.

The shielded gas is pure argon and pre-heating is required for dryingonly to produce welds of the highest quality. All surfaces and welding wireshould be degreased and the area near the joint and the welding wire shouldbe stainless steel wire brushed or scrape to remove oxide and each run

brushed before the next is laid.The angles of torch and filler rod are shown in Fig. 7.8. After

switching on the gas, water, welding current and HF unit, the arc is struckby bringing the tungsten electrode near the work (without touching down).The HF sparks jump the gap and the welding current flows. Arc lengthshould be about 3 mm. Practice starting by laying the holder on its side andbringing it to the vertical position, but using the ceramic shield as a fulcrum

can lead to damage to the holder and ceramic shield. The arc is held in one

Table 7.3 Operating data for TIG

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position on the plate until a molten pool is obtained and welding iscommenced, proceeding from right to left, the rod being fed into the forwardedge of the molten pool and always kept within the gas shield. It must not be

allowed to touch the electrode or contamination occurs. A black appearanceon the weld metal indicates insufficient argon supply.

The flow rate should be checked and the line inspected for leaks. Abrown film on the weld metal indicates presence of oxygen in the argon whilea chalky white appearance of the weld metal accompanied by difficulty incontrolling the weld indicates excessive current and overheating. The weldcontinues with the edge of the portion sinking through, clearly visible, and

the amount of the sinking which determines the size of the penetration beadis controlled by the welding rate.

7.3. METAL INERT GAS (MIG)

It is convenient to consider, under this heading, those applications

which involve shielding the arc with argon, carbon dioxide (CO2) andmixtures of argon with oxygen and/or CO2, since the power source and

30o

15o

Direction oftravel

Figure 7.8. Example of TIG

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equipment is essentially similar except for gas supply. With the tungsteninert gas shielded arc welding process, inclusions of tungsten becometroublesome with currents above 300 A. The MIG process does not suffer

from these advantages and larger welding current giving greater depositionrates can be achieved. The process is suitable for welding aluminium,magnesium alloys, plain and low-alloy steels, stainless and heat-resistantsteel, copper and bronze, the variation being filler wire type of gas shieldingthe arc.

The consumable electrode of bare wire is carried on the spool and is fed

to a maually operated or fully automatic gun through an outer flexible cableby motor-driven rollers of adjustable speed, and rate of burn-off of theelectrode wire must be balance by rate of wire feed. Wire feed ratedetermines the current used.

In addition, a shielding gas or gas mixture is fed to the gun togetherwith welding current supply, cooling water flow and return (if the gun is

water cooled) and a control cable from gun switch to control contractors.A d.c. power supply is required with the wire electrode connected to thepositive pole ( Fig. 7.9).

Figure 7.9 . MIG welding equipment

Spool ofelectrodewire

Control headforelectrode feedand gas supply

Inert gascylinder

Electrodefeedrools

Weldingpowercable

Arc weldingpower supply

Gas flowmeter

Contactor lead,weldingcurrent,electrode, andinert gasto weldinggun

Contactor cable

Groundcable

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During this process an electric arc is used to heat the weld zone. Theelectrode is fed into the weld pool at a controlled rate and the arc is shieldedby a protective gas such as argon, helium, or carbon dioxide (Fig. 7.9). Gas

metal-arc welding can be either the short-circuiting process or the spray-arcprocess (Fig. 7.10).

The short-circuiting arc process (short arc) operates at low currentsand voltages. For example, 18-gauge sheet metal can be welded at 45 ampsand 12 volts.

In contrast, the spray-arc process uses high currents and voltages, e.g.,Arc action is illustrated in Fig. 7.12. This results in high heat input to the

weld area, making possible deposition rates of more than 0.4 lb per minute.(The deposition rate is the weight of filler metal melted into the weld zone

Inert/noble gas

Melting pool

ArcShielded gas

Work piece

Figure 7.10. MIG in progress

Work piece

Figure 7.11. Mechanics of the short circuiting transfer process asshown between the electrode and work piece. Electrode dips into poolan average of 90 times a second

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per unit of time.) Most applications of the spray-arc process are in thickmetal fabrications, e.g., in heavy road-building machinery, ship construction,and beams for bridges.

All metal inert-gas (MIG) welding is classified as semi-automatic, since

the electrode feeds into the weld according to a preset adjustment. Aftermaking an initial adjustment, the welding operator merely moves the gunalong the joint. For effective applications, the welding operator needsinformation concerning power requirements, welding gun, selection ofshielding gas, type of filler metal, and job procedures.

7.3.1. Power Requirements.

Conventional power supplies used for shielded metal-arcwelding are not satisfactory. A welding machine designed for the MIGprocess is called a constant potential power source; it produces aconstant voltage and also permits the operator to adjust electrode feed

rates. The adjustments on the power supply are voltage, slope (limitscurrent), and wire feed rate. Welding current is established by

Work piece

Electrode maintains steady arc length

Figure 7.12. Mechanics of the spray-arc transferprocess as shown between the electrode and work

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selecting a wire feed rate. Slope adjustment to limit current is not aproblem with spray-arc type transfer. However, in short-circuiting arcprocesses, limitations on short-circuit current are essential to prevent

excessive spatter.The electrode feed mechanism, an important part of the welding

machine, consists of a storage reel for electrode wire and a power drivewhich feeds the electrode into the weld at a controlled rate.

Metal Shielding Gas Remarks

Aluminium and copper Argon + helium20-80% mixture

High heat inputMinimum of porosity

Copper Argon + nitrogen

25-30% mixture

Good heat input on copper

Carbon steelsLow alloy steels

Argon + oxygen3-5% mixture

Stabilizes arcReduces spatterCauses weld metal to flowEliminates undercutMay require electrode to

contain deoxidizers

Low alloy steels Mixture of argon,helium and carbondioxide

Increases toughness of welddeposit

Table 7.4 Shielding mixtures for MIG

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7.3.2. Selection of Gas.

The primary purpose of the inert gas is to shield the weld crater

from contamination. Shielding gas may also affect (1) the transfer ofmetal across the arc, (2) fusion and penetration, (3) the shape of welddeposit, (4) the speed of completing the weld, (5) the ability of fillermetal to flow over the surface without undercutting, and (6) the cost ofthe finished weld.

No single inert gas is satisfactory for all welding conditions.

Some specific jobs are more efficiently welded with a mixture of gases.For example, low alloy steels are welded with a mixture of argon,helium, and carbon dioxide (Table 7.4).

7.3.3. Filler Metal.

Electrodes used for filler metal with the MIG process are muchsmaller in diameter than those used with the metal-arc process. Sizesmay range from 0.4 mm to 5.5 mm in diameter. Small diameterelectrodes require high feed rates, from 100 to 1,400 inches per minute.The composition of the electrode usually matches that of the basemetal, but for welding high-strength alloys, the composition of the

electrode may vary widely from that of the base metal.For example, an aluminium-zinc-magnesium alloy (7039) is

welded with an aluminium-magnesium alloy (5356).

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7.4. JOB PROCEDURES

High-quality welds are obtained by controlling process variables which

include current, voltage, travel speed, electrode extension, cleanliness, andtype of joint.

7.4.1. Current.

Welding current varies with the melting rate of the electrode.

Extreme values of current tend to promote defects, but a high current(1.1 mm. electrode at 220 amp) reduces the drop size of the transfer,improves arc stability, and improves penetration.

7.4.2. Voltage.

With the MIG welding process, the voltage control determinesthe arc length. The higher the voltage setting, the longer the arc. Adesirable voltage range to establish a short arc is 19-22 volts; defectsare more likely to occur outside this range (Fig. 7.14).

Seve

rity

of d

efec

t (I

ncre

ase)

Seve

rity

of d

efec

t (I

ncre

ase)

Fig. 7.13. Defects related to voltage settings.

Voltage Voltage

Curve representingundercutting

Curve representingporosity

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Position of welding will determine voltage needed. For example, ahigher voltage is more desirable for flat-position welding than for vertical oroverhead welding. Table 7-5 indicates typical voltage values.

Metal Argon Helium Ar-O2 Mixture1-5%O2

CO2

Aluminium 25 30 * *

Carbon Steel * * 28 30

Low-alloy Steel * * 28 30

Stainless Steel 24 * 26 *

Nickel 26 30 * *

Copper 30 36 * *

*Not recommended.

7.4.3. Travel Speed.

After selecting a current and voltage setting, select the rate of

travel. A typical example is 0.6m – 0.76m per minute (in./min). If therate is changed more than a few mm per minute, weld quality will begreatly affected (Fig. 7.15).

Table 7-5 Typical arc voltage for MIG using drop transfer and 1/16 inchdiameter electrode.

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Position of welding will affect the travel speed. For example, ifthe weld direction is dropped 15 degrees from flat so that the positionis slightly downhill, travel speed can be increased.

7.4.4. Electrode Extension.

Electrode extension is important. The further the electrodeextends from the gun to the arc, the greater the electrical resistancebetween the output terminals. Higher resistance increases thetemperature of the electrode, and the resistance-heated electrode usesless current in the weld puddle. In the spray-arc process, the electrodeextension should be about 12 mm to 25 mm, for short-circuiting

transfer; it should be approximately half this distance.

7.5. MIG WELDING TECHNIQUES

There are three methods of initiating the arc.

Fig. 7.15. Undercutting of horizontal fillet on 6.3mm thick aluminium asaffected by travel speed. Gas metal arc process was used.

No undercut.Travel speed26 in/min

Undercutting.Travel speed32 in/min

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i. The gun switch operates the gas and water solenoids andwhen released the wire drive is switched on together withthe welding current.

ii. The gun switch operates the gas and water solenoids and

strikes the wire end on the plate operates the wire drivesand welding current (known as ‘scratch start’).

iii. The gun switch operates the gas and water solenoids andwire feed with welding current known as ‘scratch start’.

As a general rule dip transfer is used for thinner sections up to 6.4 mm

and for positional welding, whilst spray transfer is used for thicker sections.

The gun is held at an angle of 80o or slight less to the line of the weld toobtain a good view of the weld pool, and welding proceeds from right to leftwith nozzle held 6 – 12 mm from the work.

The further the nozzle is held from the work less the efficiency of thegas shield, leading to porosity. If the nozzle is held too close to the workspatter may build up, necessitating frequent cleaning of the nozzle, whileacting between nozzle and work can be caused by a bent wire guide tubeallowing the wire to touch the nozzle, or by spatter build-up short-circuitingwire and nozzle. If the wire burns back to the guide tube it may be caused by

a late start of the wire feed, fouling of the wire in the feed conduit or the feedrolls being too tight. Intermittent wire feed is generally due to insufficientfeed rolls pressure or looseness wire due to wear in the rolls. Excessivelysharp bends in the flexible guide tubes can also lead to this trouble.

Root run is performed with no weave and filler runs with as littleweave as possible consistent with good fusion since excessive weaving tendsto promote porosity. The amount of wire projecting beyond the contact tubeis important because the greater the projection, the greater the I2R effect andthe greater the voltage drop which may reduce the welding current and affect

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penetration. The least projection commensurate with accessibility to thejoint being welded should be aimed at.

Backing the strips which are welded permanently on to the reverseside of the plate by the root run are often used to ensure sound root fusion.Backing bars of copper or ceramics with grooves of the required penetrationbead profile can be used and are removed after welding. It is not necessary toback-chip the root run of the light alloys but with stainless steel this is oftendone and a sealing run put down. The importance of fit-up in securingcontinuity and evenness of the penetration bead cannot be over-emphasized.

Flat welds may be slightly tilted to allow the molten metal to flowagainst the deposited metal and thus give a better profile. If the first run hasa very convex profile poor manipulation of the gun may cause cold laps in thesubsequent run.

7.6. DIRECT CURRENT STRAIGHT POLARITY

The welding circuit shown in figure 7.16, is known as a straightpolarity circuit. It is understood that the electrons are flowing from thenegative terminal (cathode) of the machine to the electrode. The electronscontinue to travel across the arc into the base metal and to the positiveterminal (anode) of the machine.

Approximately two-thirds of the total heat produced with DCSP isreleased at the base metal while one-third is released at the electrode. Thechoice of direct current straight polarity depends on many variables such asmaterial of the base metal, position of the weld, as well as the electrodematerial and covering.

Electrode

Reactor

Cathode

FieldHolder

Anode

Arc gap

Work piece

Figure 7.16. Wiring diagram of a direct current, straight polarity (DCSP)arc circuit

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7.7. DIRECT CURRENT REVERSE POLARITY ARC WELDING

It is possible, and sometimes desirable, to reverse the direction ofelectron flow in the arc welding circuit. When electron flow from the negativeterminal (cathode) of the arc welder to the base metal, this circuit is knownas direct current reverse polarity (DCRP). In this case, the electron returnsto the positive terminal (anode) of the machine from the electrode side of thearc, as shown in Figure 7.17.

When using DCRP, one-third of the heat generated in the arc isreleased at the base-metal and two-thirds is liberated at the electrode. Withtwo-thirds of the heat released at the electrode in DCRP, the electrode metaland the shielding gas are super-heated. This superheating causes the moltenmetal in the electrode to travel across the arc at a very high rate of speed.Deep penetration results due to the force of the high velocity arc. There istheory that, with a covered electrode, a jet action and/or expansion of gases inthe metal at the electrode tip causes the molten metal to be propelled withgreat impact across the arc.

The choice of direct current reverse polarity depends on manyvariables such as material of the base metal, position of the weld, as well asthe electrode material and covering.

Anode

Electrode

Reactor

Cathode

FieldHolder

Arc gap

Work piece

Figure 7.17. Wiring diagram of a direct current, reverse polarity (DCRP)arc circuit

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7.1. Explain the term nonconsumable electrode.

7.2. What does the term inert signify?

7.3. List the gases used for shielding a welding arc.

7.4. Explain how TIG welding electrodes are shaped.

7.5. How far should the electrode extend beyond the nozzle of the TIGtorch?

7.6. Explain why MIG welding is classified as a semiautomatic process.

ACTIVITY 7

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7.1. The electrode does not melt into the weld.

7.2. The gas does not combine with the base metal or filler.

7.3. Argon, helium and carbon dioxide.

7.4. The electrode diameter should match the current value. If the current istoo high for the diameter of the electrode the life of the electrode willbe short. When the current is too low for a given electrode diameter,the arc will not be stable. The end of the electrode should remainbright, as if it was polished.

7.5. The electrode should extend beyond the nozzle a distance equal to the

electrode diameter.

7.5. MIG welding is classified as semi-automatic because the electrodefeeds into the weld according to a preset adjustment. After making aninitial adjustment, the welding operator merely moves the gun alongthe joint. For effective applications, the welding operator needsinformation concerning power requirements, welding gun, selection of

shielding gas, type of filler metal, and job procedures.

FEEDBACK ON ACTIVITY 7

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1. From the standpoint of operation, how are TIG and MIG processesdifferent? How are they similar?

2. What polarity does anode signify?

3. In what direction do the electrons travel when using straight polarity?

4. How much of the heat used for arc welding is liberated at the electrodewhen using straight polarity?

5. Why is it recommended that a tungsten electrode arc be started on ascrap tungsten surface?

6. What would happen if the tungsten electrode were bent off centre?

7. Name two defects that could occur with gas shielded-arc weldingprocesses and explain how each could be avoided.

SELF-ASSESSMENT 7

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1. TIG uses a tungsten electrode that does not melt into the weld;because the electrode is shielded and cooled by inert gas flow. Aseparate filler rod is used as needed

MIG uses a continuous electrode which feeds into the weldautomatically as an arc is maintained. . They both use inert gas.

FEEDBACK OF SELF-ASSESSMENT 7

TIG in progress. The tungsten does not melt into the puddle forfiller. This is a nonconsumable electrode.

Shielded gas

Electrode(tungsten)

Filler rode

arc

Melting pool

Inert/noblegas

Work piece

20 – 30o 80 – 90oDirection of travel

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2. Positive (+)

3. Across the arc into the base metal and to the positive terminal.

4. One-third (1/3)

5. To keep the tungsten electrode clean.

6. Uses more current and electrode will be jagged or contaminated.

7. (a) Eyes and skin – arc is more intense. Wear leather and speciallytreated cloth.(b) Breathing – provide adequate ventilation.

Inert/noble gas

Melting pool

ArcShielded gas

Work piece

MIG in progress

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Unit7 Shielded Gas Arc Welding